When profiling ngcc it is notable that a large amount of time
is spent dealing with an exception that is thrown (and handled
internally by fs) when checking the existence of a file.
We check file existence a lot in both finding entry-points
and when TS is compiling code. This commit adds a simple
cached `FileSystem`, which wraps a real `FileSystem` delegate.
This will reduce the number of calls through to `fs.exists()` and
`fs.readFile()` on the delegate.
Initial benchmarks indicate that the cache is miss to hit ratio
for `exists()` is about 2:1, which means that we save about 1/3
of the calls to `fs.existsSync()`.
Note that this implements a "non-expiring" cache, so it is not suitable
for a long lived `FileSystem`, where files may be modified externally.
The cache will be updated if a file is changed or moved via
calls to `FileSystem` methods but it will not be aware of changes
to the files system from outside the `FileSystem` service.
For ngcc we must create a new `FileSystem` service
for each run of `mainNgcc` and ensure that all file operations
(including TS compilation) use the `FileSystem` service.
This ensures that it is very unlikely that a file will change
externally during `mainNgcc` processing.
PR Close#30525
7f2330a968